(CNN) -- International inspectors have made new visits to two of Iran's nuclear facilities, including a research reactor that the U.N.'s watchdog agency has been kept out of for a year, a source familiar with Iran's International Atomic Energy Agency file said Thursday.

Inspectors visited the uranium enrichment plant at Natanz and the still-unfinished research reactor at Arak last week, the source, who was not authorized to speak publicly on the matter, told CNN. The visit marked the first time inspectors had been to Arak, and inspectors went to verify design information for the plant, the source said.

Iran says the plant is meant to produce radioactive isotopes for medical use. But since its spent fuel could be reprocessed to produce plutonium, it has been identified as a possible "proliferation concern," according to a recent report from the U.S. Congressional Research Service.

Iran has also refused a U.N. Security Council demand that it halt the production of enriched uranium at Natanz, insisting it is within its rights to produce fuel for civilian power plants.

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The IAEA inspectors who went there last week asked Iran to provide more information on its improvements to the Natanz facility, which enriches uranium for nuclear fuel, so the agency can keep tabs on how much of the element is processed, the source said.

At low concentrations, enriched uranium can be used to run power plants; extremely high concentrations are required to produce a nuclear bomb. Iranian officials said in April that about 7,000 of the planned 50,000 centrifuges used to enrich uranium at Natanz had been installed.

The United States has accused Iran of concealing a nuclear weapons program. The Islamic republic insists its program is strictly for civilian power, but a U.N. report earlier this year found that the Iranian nuclear program had "military dimensions."